1. set your arx libertatis folder in the configuration page
2. put your mod zips into a /mods folder in the arx libertatis folder
3. click the update mods button on the mods page
4. select mods you want to use
5. start game with mods

mods need a manifest.json in the root of the zip in the form of

{
  "name":"My mod",
  "description":"short description",
  "author":"your name",
  "url":"project url",
}